1. Field of the Invention
The present invention relates to an optical disc on which digital data is recorded at a high density by using a multi-level pit edge recording method, and an optical disc reproducing apparatus which decodes and reproduces the digital data from the optical disc.
2. Description of the Related Art
In recent years, as a method of recording and reproducing digital data on an optical disc at a high density, an attention is paid to a multi-level pit edge recording method which modulates positions of edges of each pit on the optical disc in multiple levels so as to record the digital data by multiple levels. An attention is also paid to Partial Response Maximum Likelihood (PRML) reproduction which is a combination of a partial response technique and a Viterbi decoding technique. Further, there is proposed a method combining the multi-level pit edge recording and the PRML reproduction. For example, this method is disclosed in "Simulation of a High Density Optical Disc System Employing Multi-level Pit Edge Recording and 2 Dimensional PRML Reproduction" Vol. 98, No. 333, pp. 7-13, MR98-30, Technical Report of the Institute of Electronics, Information and Communication Engineers (October. 1998).
FIG. 15 shows a pit arrangement of the optical disc employing the method mentioned above. In FIG. 15, on an optical disc 101, there are data areas 101a on which pits carrying the digital data thereon are formed, and reference areas 101b on which reference pit patterns for carrying various kinds of information necessary for controlling a reproduction are formed.
In each of the data areas 101a, data pits Pd are arranged on tracks of the optical disc 101 at a constant interval. In each of the data pits Pd, three-level digital data is recorded by shifting the position of the pit edge in multiple levels. Particularly, as shown in FIG. 16, in the case of recording the three-level digital data comprising (0, 1, 2), at "0", the position of the pit edge is shifted by a constant distance in a direction that a pit length is shortened, at "1", the position of the pit edge is maintained, and at "2", the position of the pit edge is shifted by a constant distance in a direction that the pit length is lengthened. In addition, in FIGS. 15 and 16, all the positions of the pit edge corresponding to three levels of the three-level digital data are illustrated in an overlapping manner for ease of understanding. Actually, one position among the illustrated positions is determined depending on the level of the three-level digital data.
In each of the reference areas 101b, there are formed synchronous pits Ps, tracking pits Pt and clock pits Pc, as shown in FIG. 15. These pits form reference pit patterns as a whole. The synchronous pits Ps are used as a reference for a synchronous control at a time of reproducing, and each has a pit length longer than the other pits. The tracking pits Pt are used as a reference for a tracking control with respect to the optical disc 101, and are arranged so that the position of each pit is shifted at a predetermined distance at a three-track period. The clock pits Pc are used as a reference for reproducing a clock signal at a time of reproducing, and each clock pit PC is arranged at the same position of each of the tracks in parallel.
When reproducing the optical disc 101a beam spot SP of a reproducing laser beam traces on a center line of two tracks shown by a broken line in FIG. 15. When the beam spot SP reaches, for example, point z in FIG. 15, four pit edges corresponding to front and rear pit edges on both adjacent tracks are read at the same time.
FIG. 17 shows a schematic structure of an optical disc reproducing apparatus to which the aforementioned multi-level pit edge recording method is applied. In FIG. 17, the optical pickup 102 irradiates the optical disc 101 with a reproducing laser beam, thereby forming the beam spot SP on the optical disc 101. The optical pickup 102 then receives the light beam reflected by the optical disc 101 and generates an analog reproduction signal. An amplifier 103 amplifies the analog reproduction signal, and an N/D converter 104 converts the analog reproduction signal into a digital signal. This digital signal is output from the A/D converter 104 as sample value series. A digital filter 105 then rectifies the waveform of the sample value series to obtain a desired frequency characteristics. A Viterbi decoding circuit 106 compares the sample value series with a reference level, thereby generating reproduction data series in which a square difference is minimized. The reproduction data series is output to a circuit in the later stage from the Viterbi decoding circuit 106.
A clock phase detecting circuit 109 extracts sample values corresponding to the clock pits Pc from the sample value series output from the digital filter 105, and calculates a phase error between the reproduction signal and a reproduction clock, thereby generating phase error signal. A clock generating circuit 108 changes the frequency of the reproduction clock in accordance with the phase error signal and outputs the reproduction clock to the A/D converter 104. Thus, the phase-synchronization between the reproduction clock and the reproduction signal can be established.
A tracking error detecting circuit 110 extracts sample values corresponding to the tracking pits Pt from the sample value series output from the digital filter 105, and calculates a tracking error in the reproduction of the optical disc 101, thereby generating a tracking error signal. A tracking servo circuit 107 controls the position of the optical pickup 102 in a radial direction of the disc in accordance with the tracking error signal.
A synchronization detecting circuit 111 extracts sample values based on the synchronous pits Ps from the sample value series output from the digital filter 105, and generates a synchronization detecting signal for synchronizing the reproducing system. A timing generating circuit 112 supplies various kinds of timing signals generated on the basis of the synchronization detecting signal to each of components of the optical disc reproducing apparatus.
In the optical disc reproducing apparatus mentioned above, since four pit edges each of which carries three digital data comprising (0, 1, 2) are simultaneously read, in the reproduction signal, nine value levels (0 to 8) can be obtained by respectively adding three values at each of four pit edges. The digital data recorded on the optical disc 101 can be reproduced from the nine value level in the reproduction signal by using the PRML method. Accordingly, in comparison with the case of recording and reproducing the digital data of two value level, it is advantageous in view of increasing a recording density of the optical disc 101.
However, in the conventional method mentioned above, since the reproduction signal is finely classified into nine value level, a difference in each of the levels is small, so that it is hard to accurately determine the level and the data error tends to be increased. In particular, when the waveform of the reproduction signal is distorted due to various kinds of reasons, the data error is significantly increased.
A tilt of an optical disc caused by a warp of the optical disc is one of the causes that the waveform of the reproduction signal is distorted. In reproducing digital data from the optical disc, keeping the relationship in position between the reproduction laser beam and the optical disc perpendicular is ideal. However, if the tilt of the optical disc is generated due to a warp of the optical disc, the relationship in position between the reproduction laser beam and the optical disc cannot be kept perpendicular. As a result, the reproduction signal is distorted due to the influence of wavefront aberration of the reproduction laser beam.
Thus, in the conventional method mentioned above, as a result that the level of the reproduction signal is finely classified, the data error is easily increased due to the influence of the tilt of the optical disc, so that there is a problem that reproduction performance of the optical disc is deteriorated.